Two wire full duplex transmission of data, as performed in prior art devices, typically relies on the use of hybrids, or on frequency multiplexing.
The use of hybrid circuits, however, is expensive and, additionally, may cause echos which must then be eliminated by the use of echo cancelling circuits. The use of frequency multiplexing for such transmission requires the separation of the two directions of transmission by the use of two different frequency bands. Accordingly, expensive filters are required. Both types of prior art systems thus include deficiencies with respect to transmission of digital signals, for example.
Various systems utilizing time division multiplexing are known.
For example, Evenchik U.S. Pat. No. 4,171,467, Eggimann et al U.S. Pat. No. 3,633,172 and Lutz et al U.S. Pat. No. 3,974,339 all pertain to communication systems. The Evenchik disclosure teaches the use of a single pair of wires to interface a data and a voice communication circuit using bipolar bit pair coding. The Eggimann patent provides time compression in address-coded signaling in which discrete information packets are time position modulated. Finally, the Lutz patent further provides two-wire time division multiplexing in a telecommunications network.
None of the references, however, provide a microprocessor based communication system having the advantages provided herein.
In providing a functional data transmission set, it is desirable to provide storage for the transmitted information both before its transmission and after its reception. For example, information from a source terminal may be entering the dataset simultaneously with the reception of information from the transmission line. The data to be transmitted must be stored until the dataset is ready to transmit. Further, the dataset must have sufficient intelligence to determine when to switch from the transmission to the receiving mode, and vice versa. Moreover, the desirable system should be able to generate control words in order to communicate with a separate similar system, and to provide error checking functions.
None of the prior art provides these features.